The present invention relates to a method for locating objects using an FMCW radar, in which:                a ramp-shaped frequency-modulated radar signal is transmitted whose modulation pattern has a plurality of successive ramps having different gradients,        received radar echoes are mixed down with the transmitted signal into a baseband,        the baseband signal is recorded ramp-by-ramp and transformed into a respective spectrum,        for each signal peak found in the spectrum, a noise estimation is carried out in order to distinguish between radar targets and noise or clutter, and        by comparing the frequency positions of the mutually corresponding signal peaks in the spectra obtained for various ramp gradients, the distances R and relative velocities v of the radar targets are determined.        
In addition, the present invention relates to a radar sensor, in particular for motor vehicles, fashioned in order to carry out this method.
In motor vehicles, FMCW radar sensors are used to acquire the surrounding traffic environment, in particular in order to locate other vehicles. The location results can be used for various assistance functions, for example an automatic distance regulation, automatic collision warning, or automatic triggering of an emergency braking process when there is an acute risk of collision.
The frequency of the baseband signal corresponds to the frequency difference between the signal transmitted at a given time and the signal received at the same time. Based on the frequency modulation of the transmit signal, this frequency difference is a function of the runtime of the signal from the radar sensor to the object and back, and thus of the distance from the object. Due to the Doppler effect, the frequency difference however also contains a portion that is caused by the relative velocity of the object. The measurement of the frequency difference at a single ramp therefore does not yet permit a determination of the distance and of the relative velocity, but rather supplies only a linear relation between these quantities. In a distance/velocity diagram (R-v diagram), this relation can be represented as a straight line. In order to obtain unambiguous values for the distance and the relative velocity, a standard type of FMCW radar works with alternating rising and falling frequency ramps. In this case, in the R-v diagram a different straight line is then obtained for each ramp, and the distance and the relative velocity of the object are given by the point of intersection of these two straight lines.
If, however, a plurality of objects are located at the same time, the frequency spectrum of the baseband signal contains a plurality of peaks for each ramp, one for each object, and in a comparison of the peaks at different ramps it can no longer unambiguously be determined which peak belongs to which object. For example, given the simultaneous location of two objects an R-v diagram is obtained having four straight lines that intersect each other. Only two of the four points of intersection indicate the distances and relative velocities of the two objects, while the two other points of intersection represent so-called “phantom targets.”
In order to remove ambiguity, in most cases at least one third frequency ramp is additionally worked with that has a different gradient and supplies a different set of straight lines in the R-v diagram. The genuine objects can then be recognized in that all three straight lines go through the same point. As the number of simultaneously located objects increases, however, the expense of resolving the ambiguities increases. Often, further frequency ramps are used to more easily resolve ambiguities.
An alternative approach to the solution of this problem has also been proposed. Here, a sequence of identical, relatively short frequency ramps, so-called “rapid chirps,” is used, which, in relation to their duration, have a large frequency sweep, and are therefore so steep that in the baseband signal the distance-dependent portion dominates, while the Doppler portion represents only a small correction. This correction is determined by following the phase shift of the baseband signal from ramp to ramp. Here, the circumstance is exploited that the phase of the baseband signal reacts relatively sensitively to a small change in the distance from the object, resulting from the relative movement of the object during the short time interval from one frequency ramp to the next.
However, because the phase shift is a periodic function of the relative velocity, the relative velocity can be unambiguously determined only if it is so small that the phase shift is smaller than a half period (i.e., less than n).
Given a use of the FMCW radar in a motor vehicle, the relative velocities can be large enough that, in order nonetheless to obtain unambiguous results, the duration and thus the frequency of repetition of the chirps has to be chosen to be very short. This not only requires more computing power, but also, due to the correspondingly short “observation duration,” a greater degree of unsharpness in the distance measurement can result, so that further measures are required in order to obtain sufficiently precise distance values.
In general, in radar location there is the problem that the received radar signals are subject to noise to a greater or lesser degree, and in addition contain radar echoes from objects that are “not of interest,” for example guardrail posts, irregularities in the roadway surface, raindrops, and the like. These undesired radar echoes, so-called “clutter,” do indeed for the most part have a smaller amplitude than the radar targets that are actually of interest, but can nonetheless make the identification of the genuine radar targets more difficult, in particular if they are situated in the location region of a plurality of radar targets having approximately equal distances and/or relative velocities.